1. Field of the Invention
The present invention relates to methods and devices utilized in a wireless communication system, and more particularly, to methods and devices utilized in the wireless communication system for controlling cell selection.
2. Description of the Prior Art
A System Architecture Evolution (SAE) system, set forth by 3GPP (3rd Generation Partnership Project) is a new core network architecture for UMTS (Universal Mobile Telecommunications System), addressing cost-efficient deployment and operations for mass-market usage of IP services. The main network evolutions include meeting the targets for a radio-interface of a long term evolution (LTE) system, simplified network architecture, an All-IP Network, only packet switched means for all services, no circuit switching, and support for mobility and service continuity between heterogeneous access networks, such as between 2G/3G, LTE, non-3GPP access systems.
The SAE core network includes an MME (Mobility Management Entity), an S-GW (Serving Gateway) and a P-GW (PDN gateway). The MME is a control plane entity that manages the attachment to the network, the authentication of the user equipment (UE), and interfaces an E-RAN (evolved radio access network) for the creation of relevant radio bearers.
Functions of the SAE include the Non-Access-Stratum (NAS) functions to be performed by the UE in idle mode, the NAS signalling procedures between the UE and an evolved packet core network (EPC) via an E-UTRAN (evolved UMTS radio access network) including eNBs (evolved Node-Bs), and layer 3 signalling procedures between the UE and the EPC via non-3GPP access networks.
Under NAS functions, home cell deployments are defined as a single eNB used in a building or a small group of cells, e.g. in a campus deployment. Access to home cells is restricted to particular subscribers. In home cell deployments, a CSG (closed subscriber group) cell as part of the PLMN (public land mobile network) is only suitable for a UE if an id of the CSG cell is in a whitelist of the UE. Coverage of the CSG cell may be home, building, campus coverage, etc. The whitelist is a list of CSG cell ids where the UE is allowed to camp.
For home cell deployments, two options A and B provide different usage of the whitelist. In addition, an attach procedure and a tracking area updating (TAU) procedure, are both EMM (EPS Mobility Management) specific procedures as part of the NAS signalling procedures, and can be used for updating the whitelist of the UE.
In the option A, a CSG area is defined as a tracking area (TA) that consists of a cell or group of cells to which access is restricted to a defined group of users. A single cell can only belong to one TA or one CSG-TA. To achieve a concept of “registration to multiple tracking areas”, the MME can assign several TAs and CSG-TAs to the UE. As part of the attach and/or TAU procedures, the MME may provide the UE with one or more TACs (TA cells) identifying a CSG or non-CSG tracking area. If the TA identifies a CSG, the UE adds this identity to the whitelist. The MME indicates the UE whether or not a CSG, to which the UE is subscribed, belongs to a current registration area assigned to the UE, and if it does, the UE does not trigger the tracking area update when entering the CSG.
In addition, the MME checks if the CSG TA identifier belongs to the ones subscribed by the UE or not. If the CSG TA identifier does not match with the subscribed ones, the MME rejects the network access initiated by the UE.
In the option B, a CSG area consists of a cell or group of cells to which access is restricted to a defined group of users. A single cell can only belong to one CSG area or one non-CSG area. To achieve a concept of “registration to multiple tracking areas”, the MME assigns several TAs to the UE. The CSG cell area identity consists of a TAI+a cell identity code. The list of allowed CSG cell area identities (whitelist) shall be stored in the UICC. The MME may provide the whitelist in, for example, a similar way as today's Equivalent PLMN list or the tracking area list (e.g., acceptance message of the attach/tracking area updating procedures). The UE considers its whitelist as valid until it receives a new list in the next EMM procedure or it is commanded by the network to delete all entries in the whitelist. The UE shall treat all CSG cells where the UE does not belong to the associated CSG as though they are NOT a suitable cell.
An eNB, which belongs to a CSG area, sends the CSG cell area identity, i.e., TAI plus (parts of) cell identity, to the MME whenever authorization needs to be checked in the MME. On receipt of a new cause value the UE removes CSG cell area identity (outdated information) from the stored whitelist. This prevents non-allowed UEs from accessing cells indicated as restricted.
In the option A, the whitelist consists of CSG-TAs, whereas the whitelist in the option B consists of cell identities including TAs and cell Ids.
In the prior art, two problems of cell selection are described as follows.
The first problem is that, in the option A or B, the whitelist is updated by the attach or tracking area updating procedure. However, the attach and tracking area updating procedures can only be initiated by the UE. The initiating criteria of the attach procedure is the UE attempting to enter an EMM-CONNECTED mode from the in an EMM-IDLE mode. The tracking area updating procedure is used for the following purposes of normal tracking area updating, periodic tracking area updating and intersystem change.
As can be seen from the above, the UE only initiates the attach or tracking area updating procedure when certain conditions are met. This means the UE cannot initiates the attach or tracking area updating procedure anytime. Besides, the network only can update the whitelist of a UE when the UE performs the attach or tracking area updating procedure.
The UE cannot access a new subscribed CSG cell immediately because the UE has to wait for completion of whitelist updating that adds this CSG cell to the whitelist through the attach procedure or tracking area updating procedure. Even the UE finds the new subscribed CSG cell is the best cell for cell reselection, this CSG cell cannot be selected because this cell is not in whitelist.
The second problem is described as bellow. According to the prior art, the cell measurement and reselection rules for macro cell are applied for CSG cell as well. Besides, the user pays less money if the UE accesses the service via the subscribed CSG cell than via a non-CSG cell. When a UE subscribed CSG cell, the UE camps on the macro cell and also stays in coverage of a CSG cell deployed in coverage of the macro cell, the UE is not able to reselect this CSG cell when none of the cell reselection criteria is met or even the UE does not measure this CSG cell since measurement criteria are not met. In this situation, the UE can only continue receiving service via the macro cells even though the UE has entered the CSG cell coverage.
In the abovementioned problems of the prior art, the UE cannot select or access a suitable CSG cell immediately when a suitable CSG cell is found or subscribed. The communication quality is not optimized.
In addition to the abovementioned problems, the whitelist of the UE possibly includes a listed CSG cell that is not subscribed by the UE. The related specification does not specify actions of dealing with the whitelist. Furthermore, the specification specifies no actions for the UE or the network about a situation where the UE camps on the listed CSG cell that is not subscribed. In this situation, the CSG cell can be misused by the non-subscriber UE.